Atom scattering from isolated adsorbates on surfaces: Rainbows, diffraction interferences, and trapping resonances

Abstract

The scattering of He atoms from a CO molecule adsorbed on a Pt surface is studied theoretically by methods that include: (1) Numerically exact solutions of the time-dependent Schr6dinger equation for the scattered wave packet; (2) The sudden approximation; (3) Classical trajectories. The methods are used to obtain detailed insight into the collision dynamics, and to predict and understand interesting features in the angular intensity distribution of the scattered atoms. The analysis and interpretation of the exact quantum results is facilitated by calculations of the probability current density of the scattered particles. Some of the main results are: (i) The angular intensity distribution exhibits nonspecular maxima of two types: Several of the peaks are rainbow effects induced by the adsorbate, while others (at angles nearer to the specular) are Fraunhofer diffraction interferences. Both types of peaks contain useful, largely complementary, information on adsorbate geometry and on the He/adsorbate interaction. (ii) The angular intensity distribution is quantitatively sensitive to the adsorbate distance from the surface, suggesting possible determination of that distance from experimental data. (iii) The corrugation due to the adsorbate leads to scattering resonances associated with temporary trapping of the scattered atom at the defect site. This is a new effect of potential importance for experimental studies of atom/defect interactions. The results obtained here suggest that He scattering from isolated adsorbates exhibits distinct, substantial effects, measurement of which should yield very useful data on the adsorbates and on their interactions with gas-phase atoms.

Role of quantum vortices in atomic scattering from single adsorbates

“…The existence of many maxima in this region at small values of def ͑peaks A) is due to the contribution of the clean Pt surface, as indicated above. The three remaining lobes correspond to peaks B and C, assigned to rainbow features in the literature, 3,5 and to the portion of the wave function trapped on the surface, labeled as D. This surface trapping in the bound channel of the Morse function takes place through a very effective z-x kinetic-energy transfer due to interaction with the boss, constituted by the adsorbed CO molecule, and has been called rainbow-enhanced trapping by Yinnon et al 3 The bottom part of Fig. 1 shows the corresponding quantum trajectories.…”

“…The twodimensional potential model is that used by Yinnon et al 3 for this type of scattering process, and that has been also recently used to perform quantum dynamical calculations. 5 It consists of the sum of a Morse function for the He-Pt͑111͒ process,…”

“…1,4 However, this interpretation, based on a hard-wall model, does not account for rainbow effects, which appear when realistic potentials are considered. 3,5 Despite exhaustive work, some controversy regarding the assignment of the dynamical origin of diffraction peaks has been unavoidable. Arguments based on classical trajectories 3 are not conclusive by their own nature.…”

“…3,5 Despite exhaustive work, some controversy regarding the assignment of the dynamical origin of diffraction peaks has been unavoidable. Arguments based on classical trajectories 3 are not conclusive by their own nature. On the other hand, those based on pure quantum calculations lack intuitive insight.…”

He scattering from random adsorbates, disordered compact islands, and fractal submonolayers: Intensity manifestations of surface disorder

“…9 On the basis of the experience gained with the SA, including tests against numerically exact calculations for several model systems, 27 we estimate that at least the main predictions of the SA calculations should be reliable for the systems studied here.…”